This invention relates to materials for current limiting devices. In particular, the invention relates to polymeric materials for current limiting devices, and the devices themselves.
Current limiting devices are used in many electrical circuit applications to protect sensitive components from high fault currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response time, alternatively known as switching time, to minimize the peak fault current that develops.
There are numerous devices that are capable of limiting the current in a circuit when a short circuit, otherwise known as a high current event, occurs. Known current limiting devices include a composite material that is a filled polymeric material that exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. Thus, the material can be referred to as a PTCR composite material. An attribute of PTCR composite material is that at a certain switch temperature the material undergoes a transformation from a basically conductive material to a generally resistive material.
In some current limiting devices, the PTCR composite material, typically polyethylene loaded with carbon black, is placed under pressure between electrodes. In operation, a current limiting device is placed in a circuit to be protected.
Under normal circuit conditions, the current limiting device is in a low resistance and highly conductive state. When a high current condition occurs, the PTCR composite material heats up through resistive heating until a temperature above the "switch temperature" is reached. At this point, the PTCR composite material's resistance changes to a switched resistance, also known as a high resistance state, and the current is limited. When the high current condition is cleared, the current limiting device cools down over a time period, which may be long, to below the switch temperature.
The current limiting device, which relies on the PTCR effect of the composite material, then returns to a highly conductive state. In the highly conductive state, the current limiting device is again capable of switching to the high resistance state in response to future high current events. It is desirable that the conductive material in a reusable current limiter device exhibit a low initial conductive condition resistance Ri and a high switched condition resistance, coupled with a large robustness that is characterized by a high number of successful repeated pulses, otherwise known as "successful shots".
Another current limiting device disclosed in U.S. Pat. No. 5,614,881, the entire contents of which are incorporated by reference, relies upon material ablation and arcing that occurs at localized switching regions in composite material. The ablation and arcing may lead to at least one of high mechanical and thermal stresses on the composite material. High mechanical and thermal stresses are of course undesirable, if not controlled.
The composite material, either a PTCR material or otherwise, after a switch cycle including ablation or arcing and returning to a normal circuit condition may further exhibit an altered resistance, such as a raised initial conductive condition resistance when compared to the initial conductive condition resistance before the high current event. This altered resistance is at least partially due to an incomplete ablation of the composite material at an interface that leaves non-conducting ablation products (ablation materials) at the interface that raise the resistance of the current limiting device. The switched conductive condition then possesses fewer electrical connections between the electrodes and the composite material due to the presence of the non-conducting ablation products at the interfaces, when compared to the initial conductive condition. The altered resistance is not desirable as the range of operation for the associated current limiting device will be changed.
Known composite materials may only exhibit satisfactory switching properties, such as a low initial conductive condition resistance and high switched resistance. The mechanical toughness of these materials is not as high as needed for some current limiting device applications, where brittleness of the composite material may limit repeated operations. Further, known composite materials for current limiting devices may exhibit satisfactory mechanical toughness and good switching properties for a first high current event. While generally acceptable for a first current limiting application, an initial conductive condition resistance R.sub.i of these composite materials will not be stable, and therefore undesirable for successive high current events.
Carbon black filled polyethylene material is used in a known current limiting device, a PTCR device available from ABB Control, Inc. (Prolim 36A Current Limiter). Tests of the carbon black filled polyethylene material were conducted to determine its ratio between R.sub.i and R.sub.sw and its robustness when used as the composite material in a current-limiting device, for example as set forth in U.S. Pat. No. 5,614,881 (using the Prolim 36A composite material instead of the composite material of U.S. Pat. No. 5,614,881). The tests were conducted by abrading the surfaces of a 3/4".times.3/4" piece of the carbon black filled polyethylene material and placing the pieces between 1/4" outer diameter electrodes under about 370 psi pressure. Pulses of about 400V, each for about 10 msec, with an amplifier capable of supplying 200 A of current were applied to the known carbon black filled polyethylene material.
The results of the test are illustrated in FIG. 1. The tests indicate that the carbon black filled polyethylene material exhibited an initial conductive condition resistance, R.sub.i equal to about 0.15 ohm, a switched condition resistance Rsw equal to about 16 ohm, and a resistance ratio R.sub.i /R.sub.sw equal to about 107. The current limiter device with the polyethylene filled with carbon black material exhibited only 2 repeated pulses. These results do not lend to a successful reusable current limiter device.
Therefore, a need exists for composite materials for use in current limiting devices that are able to maintain a conductive surface at the interface, even after a high current event, without the build up of non-conducting ablation products as in prior devices, thus maintaining an initial conductive condition resistance that is generally the same as prior to the high current event. Additionally, a need exists for composite materials that possess desirable reproducible electrical and mechanical properties including a low initial conductive condition resistance, a high switched resistance, a large resistance ratio, substantially reproducible initial and switched resistances, mechanical toughness and durability, large robustness and an ability to provide a large number of repeated operations, and resistance to mechanical and thermal stresses.